This invention relates to circuits for coupling an antenna to radio frequency transmit and receive circuitry.
In radio frequency (RF) communication devices it is often desirable to minimise the complexity and number of components required for RF communication. This generally reduces the cost, power consumption and PCB area consumed by the RF front-end which handles the physical transmission and reception of RF signals. These factors are especially important in portable devices, such as mobile telephones, PDAs and laptops.
It is particularly important to minimise the number of components external to the integrated circuits around which most RF communication devices are based because such components are expensive in terms of their intrinsic cost (in comparison to equivalent devices in an integrated circuit) and the PCB area they consume. Furthermore, these components are very often outside the Built-In Self Test (BIST) and calibration routines performed internally in the integrated circuit (IC).
Typically, an RF communication device will only have a single antenna for both signal transmission and signal reception. In such devices, the transmission and reception circuitry must be able to share the antenna and this is normally achieved through the use of one or more switches between the transmission/reception circuitry and the antenna. However, aside from the additional cost and PCB area consumed through the use of external switches, the additional parasitic capacitance of the switches makes them unsuitable for very high frequency radio communications (3 to 9 GHz). In particular, insertion losses associated with external switches degrade the overall sensitivity of the receive path and increase the power consumption of the transmit path.
A prior art circuit for coupling an antenna to transmit and receive circuitry is shown in FIG. 1. Such a circuit is typically used as the radio frequency front-end between an antenna and a radio transceiver. Circuit 100 can be switched between transmit and receive modes by controlling transistors 101 and 102. When the circuit is in transmission mode, transistor 102 is used to isolate the receive path from the antenna and transistor 101 is used to amplify radio frequency transmit signal 104 for transmission at antenna 105; when the circuit is in receive mode, transistor 101 is used to isolate the transmission path and transistor 102 is used to amplify signals received at antenna 105 and provide radio frequency receive signal 103. All of the components shown in FIG. 1 are typically discrete components external to an integrated circuit comprising the radio transceiver.
A second prior art circuit for coupling an antenna to transmit and receive circuitry is shown in FIG. 2. Receive circuitry 201 and transmit circuitry 202 together comprise a radio transceiver and are typically supported at a single integrated circuit. A balun or filter 203 is used to convert balanced transmit signals from transmit circuitry 202 into a single-ended output for antenna 204, and to convert a single-ended signal received at antenna 204 into a balanced input for receive circuitry 201. Again, one or more switches are used to switch between transmit and receive modes in circuit 200. When receive circuitry 201 is active, transmit circuitry 202 is isolated from the signal path, and when transmit circuitry 202 is active, receive circuitry 201 is isolated from the signal path.
Due to the use of additional components (such as switches and filters) in the signal path, it is difficult to reduce the parasitic capacitances present in the circuits to a level suitable for use with very high frequency radio signals above 3 GHz, and in particular between 6 and 9 GHz. Furthermore, the parasitic capacitances make it difficult to achieve acceptable input and output impedance matching over a wideband frequency range. The circuits shown in FIGS. 1 and 2 are therefore unsuitable for use with ultra-wideband (UWB) radio transceivers.
Additional problems exist with deep-submicron circuits (i.e. less than around a 90 nm process) which are the becoming the standard for modern communication ICs. Conventional circuits, such as the one illustrated in FIG. 1, experience reliability problems because of the large voltage swing at the common port (the antenna 105) when transmitting. The large voltage swing directly stresses low-signal devices in the receive path such as transistor 102.
There is therefore a need for an improved circuit design suitable for fabrication using a deep-submicron process for coupling radio frequency transmit and receive circuitry to an antenna. In particular, there is a need for a circuit for coupling radio frequency transmit and receive circuitry to an antenna which can be used at very high radio frequencies and with wideband radio transceivers.